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Shaft Runout Measurement With Noncontact Displacement Sensors

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Shaft Runout Measurement With Noncontact Displacement Sensors General Sensing Application Note LA05-0022

Copyright © 2013 Lion Precision.

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Summary:

Shaft runout is a common measurement especially for condition monitoring. Capacitive and eddy-current sensors provide useful non-contact measurement solutions with distinct advantages and disadvantages.

Table of Contents

Fundamentals

shape shaft runout

Runout is the displacement of the surface of a rotating object. Non-round shafts will have significant runout by definition.

According to ASME/ANSI B5.54-2005 Methods for Performance Evaluation of Computer Numerically Controlled Machining Centers, “runout” is the total indicator reading (TIR) of an instrument measuring against a moving surface. This is usually a rotary motion and is measured for a full rotation. This means the runout value is a combination of several types of error motions, form errors, and form factors:

  • shape of the shaft
  • straightness of the shaft
  • centering errors in the location of the shaft relative to the axis of rotation (eccentricity), and
  • errors in the axis of rotation itself which itself is a product of several factors:
  • drive bearing performance
  • machine structure
  • drive alignment (tilt)
  • measuring instrument errors (indicator or sensor)
  • While techniques exist for refining a shaft runout measurement to just one or a few of these components, the purpose of this Application Note is to measure total runout with all of its contributing factors (except sensor errors). The techniques described here are intended to minimize or eliminate the sensor’s contribution to the final result. When properly applied, noncontact eddy-current and capacitive sensor measurements of shaft runout will produce results with negligible sensor errors.

    radial shaft runout

    Radial runout is perpendicular to the axis of rotation.

    Radial Shaft Runout

    Radial shaft runout is a measurement of radial displacement of the shaft surface as the shaft turns. Assuming a round shaft, contributing factors to radial runout include shaft straightness, drive/shaft alignment, bearing stiffness, and increasing runout as the bearings wear. Balance is a runout factor that is dependent on the relationships between speed and bearing stiffness and wear, and overall system stiffness. Radial shaft runout is generally used to indicate wear in the drive bearings.

    Axial Shaft Runout

    axial shaft runout

    Axial runout is measured at the center of rotation to prevent shaft end flatness/squareness errors from affecting the measurement.

    Axial shaft runout is a measurement of the axial displacement of the shaft as it rotates. This measurement is taken at the center of the shaft (on the rotary axis). Off-center measurements are called “face runout” in which the flatness and squareness of the surface become contributing factors to the measurement – factors which are not of interest in most applications. Axial shaft runout is primarily used for condition monitoring of the thrust bearing.

    Shaft Shape

    By the definition above, non-round shapes always have significant runout. An oval or hexagonal shaft which is rotating perfectly will still have significant runout as the indicator responds to radial displacements of the shaft surface due to the shaft shape.

    This Application Note assumes that the shaft being measured is round.

    Shaft Straightness

    shaft straightness

    Shaft straightness affects the runout measurement.

    Radial runout is affected by shaft straightness. If the shaft is bent, runout measurements will be dependent on the location of the measurement along the length of the shaft and the location and severity of the bend. If a shaft is fixed at both ends (e.g. between the drive and a gear box) the maximum runout will tend to be near the center. If the shaft is only fixed at the drive end (e.g. motors driving fans or propellers) the runout will tend to be worse at the floating end of the shaft.

    An otherwise straight shaft may be mounted such that the center line of the shaft is not parallel with the axis of rotation. In this case, runout measurements will depend on where the measurement is taken along the shaft.

    Synchronous and Asynchronous Shaft Runout Components

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